To the Editor:—In The Journal, March 9, page 803, Heise and Heise have suggested that a city may well offer an added measure of safety from radioactive fallout. I disagree with their meteorological interpretations and would like to explain my objections.They argue that the rising currents over a warm city account for the lower spore and mold concentration on the leeward side of the city. However, since a city is not a source of spores and molds, it is difficult to distinguish between their explanation and a simple decrease in concentration farther from the source. Besides, the fall velocity of radioactive particles in the first few hundred miles from a nuclear explosion is from 10 to 1,000 times greater than that of spores, molds, or pollen. The slight upward current over a city which might support spores, etc., will have no important influence on such rapidly falling radioactive